C3bi-binding protein on Candida albicans: temperature-dependent expression and relationship to human complement receptor type 3

Abstract

We investigated in detail the previously described capacity of pseudohyphae of Candida albicans to bind C3-coated particles. We show that the expression of the C3bi receptor of C. albicans was dependent upon the growth temperature of the fungi. C. albicans grown at 30°C bound strongly to EAC1423bi, whereas those cells grown at 38.5°C were completely devoid of this capacity. The molecule responsible for the attachment of EAC1423bi was heat labile and trypsin sensitive. Several, but not all, monoclonal antibodies to the a-chain of human complement receptor type 3 (CR3) stained C. albicans, and this reactivity was expressed in parallel with the capacity of C. albicans to bind EAC1423bi, i.e., both were dependent on the growth temperature of the fungi and were trypsin sensitive. In contrast to CR3, the binding of EAC1423bi to C. albicans did not require the presence of divalent cations. Rabbit immunoglobulin G antibodies directed against C. albicans inhibited the binding of EAC1423bi to C. albicans but not to human CR3. These inhibiting IgG antibodies recognized antigens expressed on the surface of pseudohyphae but not those of yeast cells. OKM-1, a monoclonal antibody to human CR3 inhibited the attachment of EAC1423bi to CR3 and also to C. albicans. OKM-1 precipitated a 130-kilodalton band from solubilized 12SI-labeled C. albicans. We conclude that the complement receptors on C. albicans and human CR3 were antigenically related but not identical and that they differed in their functional characteristics. Various human cells express, on their surfaces, molecules which are able to bind fragments of complement component 3, like C3b, C3bi, and C3dg. Whereas C3b binds well to complement receptor type 1 (CR1) (1, 8), membrane cofactor protein (3), and decay-accelerating factor (18) and binds weakly to CR3, C3bi can be bound by CR3 (33), CR4 (23a) and, with lower affinity, can be bound by CR1 and CR2. C3dg and C3dboth show a high affinity for CR2 (10, 32), and C3dg binds, although less strongly, to CR3. These C3binding membrane molecules differ in their respective functions. CR1 is important for the binding and clearance of C3-coated immune complexes (20) and seems to contribute to the phagocytosis of some complement-coated microorganisms (34, 35). CR3 and CR4 are the most important receptors for the ingestion of complement-coated particles by phagocytes (31). Decay-accelerating factor seems to protect cells from an attack by homologous complement (21), and CR2 acts as a receptor for growth-promoting signals on B cells (9, 22, 24, 25). C3-binding structures are not specific for human cells or mammalian cells in general but are also found in microorganisms pathogenic for humans. Mammalian cells infected with herpes simplex virus type 1 express, on their surface, glycoprotein C (gC) which is able to bind C3b (12). gC destabilizes in vitro the C3 convertase of the alternative pathway, suggesting that this gC might inhibit complement activation on the cell surface and thus protect the infected cell from complement-mediated lys...

The influence of subinhibitory concentrations of conventional and experimental antifungal drugs on the expression of the iC3b binding protein in Candida albicans strains during filamentation

“…Rosette formation of C. albicans CBS 5982 was very poor at the temperature applied, even in the absence of antifungal drugs, as expected [17,21]. Consequently, very low drug concentrations were su¤cient to abolish rosette formation completely.…”

“…The expression of an iC3b binding protein on pseudomycelial forms of C. albicans was reported by Heidenreich and Dierich [11] and subsequently con¢rmed not only on the mycelial form [21,27], but also on yeasts [28,29]. However, the investigators of the latter studies noted that the expression of this protein was markedly higher on the mycelial form than on yeasts.…”

“…In spite of the fact that the tested Candida strains were not uniformly sensitive to the antifungal drugs, big di¡erences in the inhibition of rosette formation were not observed, with the exception of the C. albicans K10 and CBS 5982 strains. However, the latter strain has previously been shown to possess a decreased ability to form rosettes when cultivated at 37³C, as the expression of the iC3b binding protein is not only dependent on morphology but also on temperature [17,21].…”

“…Rosette formation was performed as described previously [21]. Brie£y, sheep erythrocytes were coated with amboceptor (anti-erythrocyte antibody, generating an erythrocyte-antibody complex, named EA) and complement, leading to EAiC3b.…”

“…IC 95rosette formation = the lowest concentration of the antifungal agent at which C. albicans is still able to form rosettes similar to the control. a Strain with decreased ability to form rosettes, when cultivated at 37³C [17,21]. b Strain which does not produce a mycelial form and is therefore unable to form rosettes.…”